Abstract:

An electric motor drive device has an inverter adjusting the voltage
applied to an AC electric motor so as to drive the AC electric motor, a
capacitor which is charged by a current supplied from a DC power supply
supplying DC voltage between a neutral point at which a plurality of
coils of the AC electric motor are connected and a positive rail or
negative rail of an inverter and passing through the inverter, and a
control circuit controlling the inverter so that the AC electric motor
turns at a designated speed. Further, the control circuit selectively
uses field weakening control and voltage boosting control for control of
the inverter according to the conditions of the induced voltage generated
at the AC electric motor, DC power supply, and voltage of the capacitor.

Claims:

1. An electric motor drive device driving an AC electric motor including a
plurality of coils connected in a star configuration, comprisingan
inverter including a plurality of arms, each of the plurality of arms
corresponding to any one of the plurality of coils of the AC electric
motor and including a first switching device connected to a positive rail
and a second switching device connected in series between the first
switching device and a negative rail, wherein, for each of the plurality
of arms, the midpoint of the first switching device and second switching
device is connected with one end of the corresponding coil of the AC
electric motor;a capacitor charged by a current supplied from a DC power
supply supplying DC voltage between a neutral point at which the other
ends of the coils of the AC electric motor are connected and a positive
rail or negative rail of the inverter and passing through the inverter;
anda control circuit controlling the switching devices of the inverter so
that the AC electric motor rotates at a designated speed,wherein said
control circuit uses field weakening control, which lowers the induced
voltage to control the switching devices of the inverter, when the
induced voltage generated at the AC electric motor is larger than the
power supply voltage supplied by the DC power supply and, on the other
hand, uses voltage boosting control, which changes the amount of charging
of the capacitor so as to change the voltage applied to the AC electric
motor to control the switching devices of the inverter, when the induced
voltage is smaller than the power supply voltage.

2. An electric motor drive device as set forth in claim 1, wherein said
control circuit uses predetermined control among said field weakening
control and said voltage boosting control so as to control said switching
devices of the inverter when a difference of said power supply voltage
and said induced voltage is within a predetermined range.

3. An electric motor drive device as set forth in claim 1, whereinsaid
device further comprises an ammeter measuring a current flowing through
said AC electric motor, and,when the difference of said power supply
voltage and said induced voltage is within a predetermined range, said
control circuit uses said field weakening control to control the
switching devices of the inverter if the value of the current is higher
than a predetermined value and uses said voltage boosting control to
control the switching devices of the inverter if the value of the current
is said predetermined value or less.

4. An electric motor drive device as set forth in claim 1, wherein when
said induced voltage is smaller than said power supply voltage, said
control circuit uses said field weakening control to control the
switching devices of the inverter if a total of said power supply voltage
and said induced voltage is lower than the voltage across the terminals
of said capacitor.

5. An electric motor drive device as set forth in claim 4, wherein said
control circuit estimates said power supply voltage by dividing the
voltage across the terminals of said capacitor by a ratio of a time
period during which current flows from said inverter to said capacity in
one period of a switching signal for control of the switching devices of
said inverter.

6. An electric motor drive device as set forth in claim 1, whereinsaid
device further comprises a status signal acquiring unit which acquires a
status signal expressing a state of an operating environment of said AC
electric motor, and whereinsaid control circuit uses values corresponding
to the status signal and designated speed among the values giving a
predetermined indicator of induced voltage for a combination of the
status signal and speed so as to estimate the induced voltage.

7. A control method of an electric motor drive device driving an AC
electric motor including a plurality of coils connected in a star
configuration,said electric motor drive device includingan inverter
including a plurality of arms, each of the plurality of arms
corresponding to any one of the plurality of coils of the AC electric
motor and including a first switching device connected to a positive rail
and a second switching device connected in series between the first
switching device and a negative rail, wherein for each of the plurality
of arms, the midpoint of the first switching device and second switching
device is connected with one end of the corresponding coil of the AC
electric motor anda capacitor charged by a current supplied from a DC
power supply supplying DC voltage between a neutral point to which the
other ends of the coils of the AC electric motor are connected and a
positive rail or negative rail of the inverter and passing through the
inverter;said control method comprising:comparing the induced voltage
generated at said AC electric motor with the power supply voltage
supplied by said DC power supply andusing field weakening control, which
lowers the induced voltage to control the switching devices of the
inverter so that the AC electric motor rotates at a designated speed,
when the induced voltage generated at the AC electric motor is larger
than the power supply voltage supplied by the DC power supply and, on the
other hand, using voltage boosting control, which changes the amount of
charging of the capacitor so as to change the voltage applied to the AC
electric motor to control the switching devices of the inverter, when the
induced voltage is smaller than the power supply voltage.

8. An electrically driven device comprising:an AC electric motor including
a plurality of coils connected in a star configuration,an electric motor
drive device as set forth in claim 1 which controls said AC electric
motor, andan operating unit performing predetermined work using power
supplied from said AC electric motor.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001]The applicant claims the right to priority based on Japanese Patent
Application No. 2009-183363, filed on Aug. 6, 2009. The entire content of
Japanese Patent Application No. 2009-183363 is hereby incorporated by
reference.

TECHNICAL FIELD

[0002]The present invention relates to an electric motor drive device
controlling an electric motor driven by multi-phase AC electric power, a
control method of an electric motor drive device, and an electrically
driven device in which such an electric motor drive device is assembled.

BACKGROUND ART

[0003]In the past, the art has been developed of using an inverter to
convert DC electric power to three-phase AC electric power and using that
three-phase AC electric power to drive an AC electric motor. As one
example of the drive method of an electric motor using this art, "field
weakening control" weakening the magnetic field which a field system
generates so as to lower the induced voltage generated in an electric
motor at the time of high speed operation is known. Further, as another
drive method, "voltage boosting control" charging and discharging a
capacitor connected between a positive rail and negative rail of an
inverter, that is, making the voltage between the positive rail and
negative rail of the inverter fluctuate, is known. Further, to improve
the energy conversion efficiency of an electric motor, the art of
switching between these two control methods has been proposed (for
example, see Japanese Unexamined Patent Publication (Kokai) No.
2007-159368 and Japanese Unexamined Patent Publication (Kokai) No.
2006-311768).

[0004]In the art disclosed in Unexamined Patent Publication (Kokai) No.
2007-159368 and Japanese Unexamined Patent Publication (Kokai) No.
2006-311768, a circuit part for charging the capacitor becomes necessary
separate from the circuit part for driving the electric motor. For this
reason, the number of parts included in the circuit becomes greater, so
this is not preferred. As opposed to this, the art of connecting a power
supply between a neutral point of an electric motor and a negative rail
of an inverter and utilizing the current flowing through the electric
motor and inverter when the electric motor is operating so as to convert
the electric power and thereby eliminating the need for a boosting
circuit has been proposed (for example, see Japanese Unexamined Patent
Publication (Kokai) No. 10-337047).

SUMMARY OF THE INVENTION

[0005]However, in the art disclosed in Japanese Unexamined Patent
Publication (Kokai) No. 10-337047, the same switch circuit is used for
voltage boosting control and electric motor drive control, so depending
on the drive conditions of the electric motor, the energy conversion
efficiency was liable to end up failing.

[0006]As one aspect of the present invention, there is provided an
electric motor drive device which drives an AC electric motor including a
plurality of coils connected in a star configuration. Such an electric
motor drive device includes an inverter including a plurality of arms,
each of the plurality of arms corresponding to any one of the plurality
of coils of the AC electric motor and including a first switching device
connected to a positive rail and a second switching device connected in
series between the first switching devices and a negative rail, wherein,
for each of the plurality of arms, the midpoint of the first switching
device and second switching device is connected with one end of the
corresponding coil of the AC electric motor; a capacitor charged by a
current supplied from a DC power supply supplying DC voltage between a
neutral point to which the coils of the AC electric motor are connected
and a positive rail or negative rail of the inverter and passing through
the inverter; and a control circuit controlling the switching devices of
the inverter so that the AC electric motor rotates at a designated speed.
Further, the control circuit uses field weakening control, which lowers
the induced voltage to control the switching devices of the inverter,
when the induced voltage generated at the AC electric motor is larger
than the power supply voltage supplied by the DC power supply and, on the
other hand, uses voltage boosting control, which changes the amount of
charging of the capacitor so as to change the voltage applied to the AC
electric motor to control the switching devices of the inverter, when the
induced voltage is smaller than the power supply voltage. Due to this
configuration, this electric motor drive device can reduce in the number
of parts while improving the energy conversion efficiency.

[0007]Further, preferably the control circuit uses predetermined control
among the field weakening control and the voltage boosting control so as
to control the switching devices of the inverter when a difference of the
power supply voltage and the induced voltage is within a predetermined
range. When the difference between the power supply voltage and the
induced voltage is small, the higher energy conversion efficiency control
among the field weakening control and voltage boosting control is
determined by the assembly error of the AC electric motor and other
factors. Therefore, by investigating which control among field weakening
control and voltage boosting control will give a higher energy conversion
efficiency when the difference between the power supply voltage and the
induced voltage is within a predetermined range, this electric motor
drive device can suitably select the control giving the higher energy
conversion efficiency.

[0008]Further, preferably the electric motor drive device further includes
an ammeter which measures the current flowing through the AC electric
motor, and, when the difference between the power supply voltage and the
induced voltage is within a predetermined range, the control circuit uses
field weakening control to control the switching devices of the inverter
in the case where the value of the current is higher than a predetermined
value and uses voltage boosting control the switching devices of the
inverter in the case where the value of the current is that predetermined
value or less. Due to this, this electric motor drive device can select
the control method based on the results of measurement of the current
actually flowing through the AC electric motor, so it is possible to
select a higher efficiency control method in real time.

[0009]Furthermore, preferably the control circuit uses field weakening
control to control the switching devices of the inverter when the induced
voltage is smaller than the power supply voltage and the total of the
power supply voltage and induced voltage is lower than the voltage across
the terminals of the capacitor. Due to this, this electric motor drive
device selects field weakening control when the amount of charging of the
capacitor just increases even when raising the voltage supplied to the AC
electric motor, so can improve the energy conversion efficiency more.

[0010]Furthermore, preferably the control circuit estimates the power
supply voltage by dividing the voltage across the terminals of the
capacitor by a ratio of a time period during which current flows from the
inverter to the capacitor in one period of switching signals for control
of the switching devices of the inverter. Due to this, this electric
motor drive device need not be provided with a sensor for measuring the
power supply voltage. Note that, the control circuit may also multiply
the ratio of the time period during which current flows from the inverter
to the capacitor in one period of the switching signals to the power
supply voltage so as to estimate the voltage across the terminals of the
capacitor.

[0011]Furthermore, preferably the electric motor drive device further
includes a status signal acquiring unit which acquires a status signal
expressing a state of the operating environment of the electric motor
drive device, and wherein the control circuit uses values corresponding
to the status signal and designated speed among the values giving a
predetermined indicator of induced voltage for a combination of the
status signal and speed so as to estimate the induced voltage. Due to
this, this electric motor drive device can accurately estimate the
induced voltage even when the operating environment of the AC electric
motor changes.

[0012]Furthermore, as another aspect of the present invention, there is
provided a control method of an electric motor drive device for driving
an AC electric motor which includes a plurality of coils connected in a
star configuration. The electric motor drive device to which this control
method is applied includes an inverter including a plurality of arms,
each of the plurality of arms corresponding to any one of the plurality
of coils of the AC electric motor and including a first switching device
connected to a positive rail and a second switching device connected in
series between the first switching devices and a negative rail, wherein,
for each of the plurality of arms, the midpoint of the first switching
device and second switching device is connected with one end of the
corresponding coil of the AC electric motor, and a capacitor charged by a
current supplied from a DC power supply supplying DC voltage between a
neutral point to which the other ends of the coils of the AC electric
motor are connected and a positive rail or negative rail of the inverter
and passing through the inverter. Further, this control method includes
comparing the induced voltage generated at the AC electric motor with the
power supply voltage supplied by the DC power supply and using field
weakening control, which lowers the induced voltage to control the
switching devices of the inverter so that the AC electric motor rotates
at a designated speed, when the induced voltage generated at the AC
electric motor is larger than the power supply voltage supplied by the DC
power supply and, on the other hand, using voltage boosting control,
which changes the amount of charging of the capacitor so as to change the
voltage applied to the AC electric motor to control the switching devices
of the inverter, when the induced voltage is smaller than the power
supply voltage. By including such a routine, this method of driving an
electric motor can reduce the number of parts while improving the energy
conversion efficiency.

[0013]Furthermore, as another aspect of the present invention, an
electrically driven device is provided. This electrically driven device
includes an AC electric motor in which a plurality of coils are connected
in a star configuration, any one of the above electric motor drive
devices for controlling the AC electric motor, and an operating unit
performing predetermined work using power supplied from the AC electric
motor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]The features of the present invention shown here and other features
and advantages will be better understood by reference to the following
drawings and detailed description.

[0015]FIG. 1 is a schematic view of the configuration of an electric motor
drive device according to a first embodiment of the present invention.

[0016]FIG. 2 is an operational flow chart of drive processing of an
electric motor executed by a control circuit of the electric motor drive
device.

[0017]FIG. 3A is a timing chart depicting the relationship between a
carrier wave and the voltage control signals for the different phases.

[0018]FIG. 3B is a timing chart depicting switching signals generated
based on the carrier wave and voltage control signals depicted in FIG.
3A.

[0019]FIG. 4 is a schematic view of the configuration of an electric motor
drive device according to a second embodiment of the present invention.

[0020]FIG. 5 is a schematic view of the configuration of an electric motor
drive device according to a third embodiment of the present invention.

[0021]FIG. 6 is a schematic view of the configuration of an electric motor
drive device according to a fourth embodiment of the present invention.

[0022]FIG. 7 is a schematic view of the configuration of a vehicular
air-conditioning system having an electric compressor driven by an
electric motor controlled by an electric motor drive device according to
any of the embodiments of the present invention.

DESCRIPTION OF EMBODIMENTS

[0023]Below, referring to the drawings, embodiments of the present
invention will be explained. However, the scope of the present invention
is not limited to the following explanation and extends to equivalents of
the aspects of the invention described in the claims, it should be noted.

[0024]Below, an electric motor drive device according to a first
embodiment of the present invention will be explained. This electric
motor drive device drives an AC electric motor having a plurality of
coils. The coils are provided at a rotor or stator for giving a drive
force to the rotor and generate a rotating magnetic field by the flow of
the different phases of current of the three-phase AC. Further, this
electric motor drive device uses the results of comparison of a DC
voltage supplied between a neutral point where the plurality of coils are
connected in a star configuration and a positive rail or negative rail of
the inverter and an induced voltage generated at the AC electric motor as
the basis to select either field weakening control for reducing the
induced voltage or voltage boosting control. Here, control which
increases the current flowing in a direction giving rise to an effect
similar to weakening the magnetic field generated by the field system and
therefore equivalent to field weakening control is also referred to as
"field weakening control" for convenience sake. Note that, in the present
embodiment, the AC electric motor driven by the electric motor drive
device is a three-phase synchronous electric motor. However, in another
embodiment, the AC electric motor driven by the electric motor drive
device may also be a three-phase induction electric motor. Further, in
another embodiment, the AC electric motor driven by the electric motor
drive device may also be a multi-phase synchronous electric motor or
multi-phase induction electric motor other than a three-phase one.

[0025]FIG. 1 is a schematic view of the configuration depicting the
overall configuration of an electric motor drive device 1 according to a
first embodiment of the present invention. The electric motor drive
device 1 has an inverter 3, a capacitor 4, voltmeters 5, 6, and a control
circuit 7 for driving an electric motor 10.

[0026]Further, to supply the DC electric power for driving the electric
motor 10, the electric motor 10 is connected to a DC power supply 2.
Specifically, an anode of the DC power supply 2 is connected to a neutral
point of the electric motor 10 at which the different phases of coils
L1 to L3 provided at the stator are connected in a star
configuration, while the cathode of the DC power supply 2 is connected to
the negative rail 35 of the inverter 3 and grounded. Note that, as the DC
power supply 2, various power supplies supplying DC electric power may be
utilized. For example, as the DC power supply 2, a lead storage battery,
lithium ion battery, or nickel-hydrogen battery may be used.

[0027]The inverter 3 converts DC electric power supplied from the DC power
supply 2 and input through the electric motor 10 to three-phase AC
electric power in accordance with a control signal input from the control
circuit 7. For that reason, the inverter 3 has three sets of arms 31, 32,
and 33 corresponding to the U-phase, V-phase, and W-phase of the electric
motor 10. These arms 31, 32, and 33 are connected in parallel. Further,
the arm 31 has series connected switching devices SW1 and SW4. Similarly,
the arms 32 and 33 respectively have series connected switching devices
SW2 and SW5 and switching devices SW3 and SW6. Note that, here, for
convenience, the switching devices SW1 to SW3 are called the "top
switching devices", while the switching devices SW4 to SW6 are called the
"bottom switching devices".

[0028]The switching devices SW1 to SW6, for example, have transistors and
diodes connected in parallel to the transistors so that current flows in
the opposite direction to the direction of flow of current through the
transistors (for example, the emitter terminals of the NPN type
transistors and the anode terminals of the diodes are connected and the
collector terminals of the transistors and the cathode terminals of the
diodes are connected). Note that, as the transistors of the switching
devices, for example, insulated gate bipolar transistors (IGBT) or power
MOSFETs (metal oxide semiconductor field effect transistors) can be
utilized. Note that, the switching devices SW1 to SW6 may also have
another configuration enabling switching on/off in accordance with the
rotational speed of the electric motor 10.

[0029]Further, for the arm 31, one end of the switching device SW1
(cathode terminal side of diode) is connected with the positive rail 34
of the inverter 3. On the other hand, the other end of the switching
device SW1, that is, the midpoint of the switching devices SW1 and SW4,
is connected with the U-phase terminal of the electric motor 10. Further,
one end of the switching device SW4 (anode terminal side of diode) is
connected with the negative rail 35 of the inverter 3. Similarly, for the
arm 32, one end of the switching device SW2 is connected to the positive
rail 34 of the inverter 3. On the other hand, the midpoint of the
switching devices SW2 and SW5 is connected to the V-phase terminal of the
electric motor 10. Further, one end of the switching device SW5 is
connected to the negative rail 35 of the inverter 3. Furthermore, for the
arm 33, one end of the switching device SW3 is connected with the
positive rail 34 of the inverter 3. On the other hand, the midpoint of
the switching devices SW3 and SW6 is connected to the W-phase terminal of
the electric motor 10. Further, one end of the switching device SW6 is
connected to the negative rail 35 of the inverter.

[0030]Further, the switching devices SW1 to SW6 are switched on or off by
a control signal from the control circuit 7. Further, for example, by the
top switching device of any one arm and the bottom switching devices of
the other arms turning on and the other switching devices turning off,
current flows to the electric motor 10 and therefore the electric motor
10 turns. Further, by successively changing the on switching devices for
the arms, the electric motor 10 stably turns by a designated speed.

[0031]The capacitor 4 uses the current flowing to the electric motor 10 to
boost the voltage between the positive rail 34 and the negative rail 35
and drive the electric motor. For that purpose, the capacitor 4 is
charged or discharges by the voltage supplied from the DC power supply 2
through the electric motor 10 and inverter 3. For that reason, one end of
the capacitor 4 is connected to the positive rail 34 of the inverter 3,
while the other end of the capacitor 4 is connected to the negative rail
35 of the inverter 3. Further, when the electric motor 10 is controlled
by voltage boosting control, the capacitor 4 is charged or discharged in
accordance with the voltage supplied through the inverter 3. Due to this,
the voltage between the positive rail 34 and negative rail 35 of the
inverter 3 is boosted and the electric motor 10 driven. On the other
hand, when the electric motor 10 is controlled by field weakening
control, the capacitor 4 is charged and discharged so that the voltage of
the capacitor 4 is maintained at a constant voltage and thereby the
electric motor 10 is driven. Note that, the capacitor 4 may be made, for
example, an electrolytic capacitor, a film capacitor, or a ceramic
capacitor.

[0032]The voltmeter 5 measures the voltage Vdc across the terminals
of the capacitor 4. Further, the voltmeter 5 notifies that measurement
voltage Vdc to the control circuit 7. Further, the voltmeter 6
measures the power supply voltage of the DC power supply 2, that is, the
voltage VL between the neutral point of the electric motor 10 and
the negative rail 35 of the inverter 3. Further, the voltmeter 6 notifies
that measurement voltage VL to the control circuit 7. Note that,
instead of having the voltmeter 5 directly measure the voltage Vdc
across terminals of the capacitor 4, it is also possible to use a
voltmeter to measure the voltage VH between the neutral point of the
electric motor 10 and the different phases of terminals, that is, the
neutral point of the electric motor 10 and the positive rail of the
inverter 3, and have the control circuit 7 calculate the voltage Vdc
across the terminals of the capacitor 4 in accordance with the following
formula:

Vdc=VH+VL (1)

[0033]The control circuit 7 controls the switching devices SW1 to SW6 of
the inverter 3 so as to make the electric motor 10 turn by a rotational
speed instructed from another device. Further, the control circuit 7
selects either voltage boosting control or field weakening control based
on the induced voltage estimated from the instructed rotational speed and
the measurement voltages Vdc, VL of the voltmeters 5, 6 so as
to improve the energy conversion efficiency of the electric motor 10. For
that reason, the control circuit 7 has a sensor signal receiving unit 11,
an interface unit 12, a control unit 13, a pulse waveform generation unit
14, and a pulse output unit 15.

[0034]The sensor signal receiving unit 11 has an interface circuit for
connecting the control circuit 7 with the voltmeters 5, 6. Further, the
sensor signal receiving unit 11 receives measurement voltages from the
voltmeters 5, 6 and transfers the measurement voltages to the control
unit 13.

[0035]The interface unit 12 has an interface circuit for connecting the
control circuit 7 with another control device controlling the apparatus
in which the electric motor 10 is carried, for example, a control device
of a vehicle in which the electric motor 10 is carried, through a
communication network. Further, the interface unit 12 transfers a control
signal received through that communication network, for example, a speed
instruction signal instructing the target speed of the electric motor 10,
a start instruction signal instructing the start of the electric motor
10, or a stop instruction signal making the electric motor 10 stop, to
the control unit 13.

[0036]The control unit 13 has one or more processors, a memory, and
peripheral circuits. Further, the control unit 13 controls the switching
devices SW1 to SW6 of the inverter 3 in accordance with the control
signal received through the interface unit 12 so as to drive the electric
motor 10. Further, the control unit 13 selects either voltage boosting
control or field weakening control based on the speed designated by the
speed instruction signal and the measurement voltages Vdc, VL
of the voltmeters 5, 6 so as to raise the energy conversion efficiency of
the electric motor 10 as much as possible. Further, the control unit 13
determines the timings of turning the switching devices SW1 to SW6 of the
inverter 3 on or off in accordance with the selected control method and
speed designated by the speed instruction signal so as to adjust the
drive current flowing through the electric motor 10.

[0037]FIG. 2 is an operational flow chart of drive processing of the
electric motor executed by the control unit 13. The control unit 13
repeatedly executes the processing illustrated by this operational flow
chart by a preset predetermined period. First, the control unit 13
acquires the voltage Vdc across terminals of the capacitor and the
power supply voltage VL from the voltmeters 5, 6 through the sensor
signal receiving unit 11 (step S101). Further, the control unit 13
detects the mechanical angular speed ωrm (rpm unit) of the
electric motor 10 designated by the speed instruction signal received
through the interface unit 12 (step S102).

[0038]Next, the control unit 13 estimates the induced voltage VE
generated at the electric motor 10 from the mechanical angular speed
ωrm (step S103). For example, the control unit 13 calculates
the induced voltage VE in accordance with the following formula.

Here, KE is an induced voltage constant which is found in advance in
accordance with the configuration of the electric motor 10. Further,
ωre is the electrical angular speed (rad/sec unit).
Furthermore, p is the number of pole pairs of the electric motor 10 (that
is, the number of poles formed in the field system of the electric motor
10 divided by two).

[0039]The control unit 13 judges if the absolute value of the difference
between the power supply voltage VL and the induced voltage VE
is less than a predetermined value Voff (step S104). Note that, the
predetermined value Voff is, for example, set to a value whereby the
difference of the power supply voltage VL and the induced voltage
VE, when becoming 0 for an ideal electric motor 10, becomes the
maximum value of the voltage difference occurring due to allowable
manufacturing differences in the electric motor 10, that is, a value of 0
or more. When, at step S104, the absolute value of the difference between
the power supply voltage VL and the induced voltage VE is the
predetermined value Voff or more, the control unit 13 judges if the
induced voltage VE is the value of the power supply voltage VL
plus a predetermined offset value Voff2 or more (step S105). Note
that, the predetermined offset value Voff2 is, for example,
determined in accordance with the characteristics of the electric motor
10 and may be either a positive value or negative value. Note that, if
the electric motor 10 is an ideal electric motor free of any influences
due to manufacturing variations, the offset value Voff2 is set to 0.
When the induced voltage VE is (VL+Voff2) or more, the
control unit 13 selects field weakening control (step S106). In this
case, even if raising the voltage VH between the positive rail 34 of
the inverter 3 and the neutral point of the electric motor 10, it is not
possible to make the power supply voltage VL higher than, the
induced voltage VE, so the balance between the voltage between the
neutral point and positive rail and voltage between the neutral point and
negative rail ends up being lost. For this reason, voltage fluctuation
ends up being caused and the energy conversion efficiency falls.
Therefore, the control unit 13 selects field weakening control.

[0040]On the other hand, when, at step S105, VE is less than
(VL+Voff2), the control unit 13 judges if the voltage Vdc
across the terminals of the capacitor is higher than the value of the
induced voltage VE plus the power supply voltage VL and offset
value Voff2 (step S107). If the voltage Vdc across the
terminals of the capacitor is higher than (VE+VL+Voff2),
the control unit 13 selects field weakening control (step S106). In this
case, even if the electric motor drive device 1 uses voltage boosting
control, the amount of charging of the capacitor 4 will just increase.
The drive force of the electric motor 10 will also not become greater and
the energy conversion efficiency will fall. On the other hand, if the
voltage Vdc across the terminals of the capacitor is
(VE+VL+Voff2) or less, the control unit 13 selects voltage
boosting control (step S108). In this case, the control circuit 7 can use
voltage boosting control to increase or decrease the voltage Vdc
across the terminals of the capacitor 4, that is, to increase or decrease
VH, so as to change the voltage substantially applied to the
differences phases of coils L1 to L3 of the electric motor 10.

[0041]Further, when, at step S104, the absolute value of the difference of
the power supply voltage VL and the induced voltage VE is less
than the predetermined value Voff, that is, the induced voltage
VE and the power supply voltage VL are substantially equal, the
control unit 13 judges whether, for this case, the preset control method
is field weakening control (step S109). Further, if the preset control
method is field weakening control, the control unit 13 selects field
weakening control (step S106). On the other hand, if the preset control
method is not field weakening control, that is, it is voltage boosting
control, the control unit 13 selects voltage boosting control (step
S108). Note that, to determine in advance the control method to be
applied when the induced voltage VE and the power supply voltage
VL are substantially equal, for example, voltage boosting control
and field weakening control are respectively applied when the power
supply voltage VL is substantially equal to the induced voltage
VE, and the currents flowing between the different phases of
terminals of the electric motor and inverter when the electric motor is
driven at a predetermined speed are measured in advance. The smaller the
current, the smaller the amount of energy consumption as well, so the
higher the energy conversion efficiency of the electric motor. Therefore,
the control giving the smaller measurement currents is determined in
advance as the control method selected when the power supply voltage
VL, is substantially equal to the induced voltage VE. Further,
a memory of the control unit 13 stores in advance a flag for identifying
the selected control method. At the above step S109, when the power
supply voltage VL is substantially equal to the induced voltage
VE, the control unit 13 may refer to the flag stored in the memory
and select voltage boosting control or field weakening control.
Furthermore, by controlling the inverter 3 so that the power supply
voltage VL becomes substantially equal to the induced voltage
VE and measuring in advance the measurement current when applying
voltage boosting control or field weakening control while changing the
speed, the control method selected at each speed may be determined in
advance. In this case, a flag set for each speed is stored in advance in
the memory along with the corresponding speed. Further, at the above step
S109, the control unit 13 may refer to the flag corresponding to the
speed closest to the current speed among the speeds stored in the memory
so as to select the voltage boosting control or field weakening control.

[0042]After step S106 or S108, the control unit 13 determines the timings
of turning the switching devices SW1 to SW6 of the inverter on/off in
accordance with the selected control method and speed designated by the
speed instruction signal (step S110). Further, the control unit 13
outputs a voltage control signal in accordance with that timing to the
pulse waveform generation unit 14 to end the control processing.

[0043]In the present embodiment, the control circuit 7 uses pulse width
modulation (PWM) control to turn the switching devices SW1 to SW6 on/off.
Therefore, the control unit 13 designates the timings for turning the
switching devices SW1 to SW6 on/off by outputting, as voltage control
signals corresponding to the phases of the electric motor 10, period
signals for comparison with the carrier wave generated by the pulse
waveform generation unit 14, to the pulse waveform generation unit 14.
The control unit 13, for example, changes the offset value, amplitude, or
period of this period signal so as to switch between voltage boosting
control and field weakening control. Further, when controlling the
electric motor 10 by voltage boosting control, the control unit 13, for
example, changes the voltage Vdc across the terminals of the
capacitor 4 in accordance with the speed of the electric motor 10. For
this reason, the control unit 13 generates voltage control signals so as
to change the ratio of the time period where all of the switching devices
SW1 to SW3 connected to the positive rail 34 of the inverter become off
and the time period where any of the switching devices SW1 to SW3 becomes
on in accordance with the targeted voltage Vdc across the terminals
of the capacitor 4. On the other hand, when controlling the electric
motor 10 by field weakening control, the control unit 13 generates
voltage control signals so as to maintain constant the ratio of the time
period where all of the switching devices SW1 to SW3 become off and the
time period where any of the switching devices SW1 to SW3 becomes on so
that the voltage across the terminals of the capacitor 4 is held
constant.

[0044]The pulse waveform generation unit 14, for example, uses the voltage
control signals received from the control unit 13 as the basis to
generate switching signals for turning the switching devices SW1 to SW6
of the inverter 3 on/off. In the above way, in the present embodiment,
the control circuit 7 uses PWM control to turn the switching devices SW1
to SW6 on/off. Therefore, the pulse waveform generation unit 14 compares
the voltage control signals corresponding to the different phases of the
electric motor 10 received from the control unit 13 with the period
signal having a predetermined period generated by an oscillation circuit
(not shown) of the pulse waveform generation unit 14, that is, the
carrier wave, by comparison circuits (not shown). Further, for each
phase, when the voltage control signal is higher than the carrier wave,
the pulse waveform generation unit 14 generates switching signals turning
on the switching device connected to the positive rail 34 of the
corresponding arm of the inverter 3 and turning off the switching device
connected to the negative rail 35 of the corresponding arm of the
inverter 3.

[0045]FIG. 3A is a timing chart depicting the carrier wave and the
different phases of voltage control signals, while FIG. 3B is a timing
chart depicting switching signals generated based on the carrier wave and
voltage control signals shown in FIG. 3A. In FIG. 3A and FIG. 3B, the
abscissa shows the time. Further, in FIG. 3A, the sawtooth wave 301 shows
the carrier wave. Further, the sine waves 302 to 304 respectively show
the voltage control signals for the U-phase, V-phase, and W-phase.
Furthermore, in FIG. 3B, the pulse-like waves 311 to 316 show the
switching signals for the switching devices shown at the left of the
waves. For example, if comparing the initial triangle wave of the carrier
wave 301 and the voltage control signal 302 corresponding to the U-phase,
in the interval from the time t0 to t1, the carrier wave 301 is
higher than the voltage control signal 302. For this reason, in the
interval from the time t0 to t1, switching signals turning off
the switching device SW1 connected to the positive rail 34 included in
the arm 31 corresponding to the U-phase of the inverter 3 and turning on
the switching device SW4 connected to the negative rail 35 are generated.

[0046]Note that, the carrier wave is not limited to a sawtooth shaped
wave. The carrier wave may also be a wave of a voltage changing to
repeatedly monotonously increase and monotonously decrease along with the
elapse of time, for example, a sine wave. The pulse waveform generation
unit 14 transfers the switching signals to the pulse output unit 15.

[0047]The pulse output unit 15 has an interface circuit for connecting the
control circuit 7 and the switching devices of the inverter 3. Further,
the pulse output unit 15 outputs the switching signals received from the
pulse waveform generation unit 14 to the corresponding switching devices
of the inverter 3. Due to this, the control circuit 7 can turn the
switching devices SW1 to SW6 of the inverter 3 on/off at the desired
timings. For this reason, the control circuit 7 can make the electric
motor 10 turn by either voltage boosting control or field weakening
control at the speed designated by the speed instruction signal.

[0048]As explained above, the electric motor drive device according to the
first embodiment of the present invention uses the results of comparison
of the power supply voltage connected between the neutral point of the
electric motor and the negative rail of the inverter and the induced
voltage as the basis to select the voltage boosting control or field
weakening control which is higher in energy conversion efficiency. Due to
this, this electric motor drive device does not have a boosting circuit
separate from the inverter and can improve the energy conversion
efficiency of the electric motor.

[0049]FIG. 4 is a schematic view of the configuration of an electric motor
drive device according to a second embodiment of the present invention.
As shown in FIG. 4, the electric motor drive device 21 according to the
second embodiment differs from the electric motor drive device 1
according to the first embodiment in the point that one end of the
capacitor 4 is connected to the neutral point of the electric motor 10 at
which the different phases of coils L1 to L3 are connected by a
star configuration, while the other end of the capacitor 4 is connected
to the positive rail 34 of the inverter 3. However, in this case as well,
when the electric motor 10 is controlled by voltage boosting control, the
capacitor 4 is charged or discharges in accordance with the voltage
between the neutral point of the electric motor 10 and the positive rail
34 of the inverter 3 so that thereby the voltage supplied to the electric
motor 10 is adjusted. Note that, in FIG. 4, the components of the
electric motor drive device 21 are assigned reference numerals the same
as the corresponding components of the electric motor drive device 1
shown in FIG. 1.

[0050]Regarding this electric motor drive device 21 as well, the control
unit 13 of the control circuit 7 controls the electric motor 10 by
voltage boosting control or field weakening control in accordance with
the operational flow chart shown in FIG. 2. However, in the electric
motor drive device 21, the relationship of formula (1) does not stand
between the voltage Vdc across the terminals of the capacitor 4 and
the voltage VH between the neutral point of the electric motor 10
and the positive rail 34 of the inverter 3. Further, Vdc is equal to
VH itself. For this reason, in the judgment processing of step S107
in the operational flow chart shown in FIG. 2, when Vdc is larger
than the induced voltage VE, the processing of step S106 is
executed, while if Vdc is the induced voltage VE or less, the
processing of step S108 is executed.

[0051]FIG. 5 is a schematic view of the configuration of an electric motor
drive device according to a third embodiment of the present invention. As
illustrated in FIG. 5, the electric motor drive device 22 according to
the third embodiment differs from the electric motor drive device 1
according to the first embodiment in the point that the anode of the DC
power supply 2 is connected to the positive rail 34 of the inverter 3 and
the cathode of the DC power supply 2 is connected to the neutral point of
the electric motor 10 at which the different phases of coils L1 to
L3 are connected in a star configuration. Note that, in FIG. 5, the
components of the electric motor drive device 22 are assigned the same
reference numerals as corresponding components of the electric motor
drive device 1 shown in FIG. 1.

[0052]Regarding this electric motor drive device 22 as well, the control
unit 13 of the control circuit 7 controls the electric motor 10 by
voltage boosting control or field weakening control in accordance with
the operational flow chart shown in FIG. 2. However, in this electric
motor drive device 22, the voltage VH between the neutral point of
the electric motor 10 and the positive rail 34 of the inverter 3 becomes
equal to the power supply voltage of the DC power supply 2. When the
electric motor 10 is controlled by voltage boosting control, the voltage
VL between the neutral point of the electric motor 10 and the
negative rail 35 of the inverter 3 fluctuates. Further, the voltmeter 6
measures the voltage VH. For this reason, the control unit 13
switches VL and VH and executes the processing of steps S104,
S105, and S107 in the operational flow chart shown in FIG. 2.

[0053]FIG. 6 is a schematic view of the configuration of an electric motor
drive device according to a fourth embodiment of the present invention.
As illustrated in FIG. 6, the electric motor drive device 23 according to
the fourth embodiment differs from the electric motor drive device 22
according to the third embodiment in the points that one end of the
capacitor 4 is connected to the neutral point of the electric motor 10 at
which the different phases of coils L1 to L3 are connected in a
star configuration and that the other end of the capacitor 4 is connected
to the negative rail 35 of the inverter 3. However, in this case as well,
when the electric motor 10 is controlled by voltage boosting control, the
capacitor 4 is charged or discharges in accordance with the voltage
between the neutral point of the electric motor 10 and the negative rail
35 of the inverter 3 so as to thereby adjust the voltage supplied to the
electric motor 10. Note that, in FIG. 6, the components of the electric
motor drive device 23 are assigned the same reference numerals as
corresponding components of the electric motor drive device 22 shown in
FIG. 5.

[0054]Regarding this electric motor drive device 23 as well, the control
unit 13 of the control circuit 7 controls the electric motor 10 by
voltage boosting control or field weakening control in accordance with
the operational flow chart shown in FIG. 2. However, in the electric
motor drive device 23, the voltage Vdc across the terminals of the
capacitor 4 and the voltage VL between the neutral point of the
electric motor 10 and the negative rail 35 of the inverter 3 are equal.
For this reason, in the judgment processing of step S107 of the
operational flow chart shown in FIG. 2, if Vdc is larger than the
induced voltage VE, the processing of step S106 is executed, while
if Vdc is the induced voltage VE or less, the processing of
step S108 is executed. Further, the power supply voltage is VH, so
the control unit 13 switches VL and VH and executes the
processings of steps S104 and S105 of the operational flow chart shown in
FIG. 2.

[0055]Note that, in the above embodiments, when the effect of
manufacturing differences in the electric motor etc. on the estimated
value of the induced voltage is small, the processings of step S104 and
S109 may also be omitted. In this case, the offset value Voff may
also be set to 0. Further, when the drop in the energy conversion
efficiency due to the difference of the voltage across the terminals of
the capacitor minus the total of the power supply voltage and induced
voltage is small, the processing of step S107 may also be omitted.

[0056]Further, in the above embodiments, a reference table showing the
relationship between the speed of the electric motor designated by the
speed instruction signal (mechanical angular speed ωrm) and
the induced voltage corresponding to that speed may be prepared in
advance for example in accordance with results of measurement or results
of simulation. Further, that reference table may be stored in advance in
a memory of the control unit 13 of the control circuit 7. In this case,
the control unit 13 can refer to the reference table and estimate the
induced voltage corresponding to the mechanical angular speed of the
electric motor designated by the speed instruction signal. Due to this,
even when manufacturing differences etc. of the electric motor 10 mean
that the speed of the electric motor designated by the speed instruction
signal and the actually generated induced voltage do not satisfy the
relationship of formula (2), it is possible to accurately estimate the
induced voltage.

[0057]Further, in the above embodiments, rather than measuring the VL
or VH corresponding to the supplied voltage of the DC power supply,
the electric motor drive device may also use standard values of supplied
voltage of the DC power supply as VL or VH. In this case, in
the electric motor drive device, the voltmeters for measuring VL or
VH may also be omitted. Alternatively, in the above embodiments,
rather than measuring VL or VH, the electric motor drive device
may also estimate VL or VH from the voltage Vdc across the
terminals of the capacitor 4 and the switching signal output from the
control circuit 7 to the inverter 3. For example, in the first
embodiment, during one period of the switching signal, the total of the
time periods when the bottom switching devices SW4 to SW6 connected to
the negative rail 35 of the inverter 3 are on is defined as TON and
the total of the time periods when the bottom switching devices SW4 to
SW6 are off is defined as TOFF. The capacitor 4 is charged through
that bottom switching devices and the corresponding top switching devices
during the time period where the bottom switching devices are off, so
there is the following relation between Vdc and VL.

V dc = T ON + T OFF T OFF V L ( 3 )
##EQU00002##

Therefore, the control unit 13 can acquire the switching signals from the
pulse waveform generation unit 14, find the time periods TON and
TOFF from the switching signals, and enter into formula (3) these
time periods and the speed designated by the speed instruction signal and
the measurement voltage Vdc so as to estimate the VL.
Alternatively, by entering the measured values of VL or VH or
the prescribed value of the power supply voltage corresponding to VL
or VH into the formula (3), the control unit 13 can estimate the
voltage Vdc across the terminals of the capacitor 4. In this case,
the voltmeter for measuring the voltage Vdc across the terminals of
the capacitor 4 may be omitted.

[0058]Furthermore, the control unit 13 may also use a parameter defining
the voltage control signals instead of the time periods Ton and
Toff to estimate VL and VH. In the above, the switching
signals are determined by the result of comparison of the carrier wave
generated by the pulse waveform generation unit 14 and the voltage
control signals. For this reason, if the carrier wave is fixed, the time
periods Ton and Toff are also unambiguously determined in
accordance with the voltage control signals. That is, the control unit 13
can use parameters defining the voltage control signals instead of the
time periods Ton and Toff to estimate VL and VH. For
example, for the first embodiment, the control unit 13 can calculate
VL by the following formula where the magnitude of the voltage
vector corresponding to the voltage control signal for each phase is made
"V":

VL=A×(Vdc-V)

[0059]where, A is a correction coefficient. Further, the voltage vector
can be made the effective value or wave height value of the voltage
control signals if, for example, the period and amplitude of the voltage
control signals are determined.

[0060]Further, the electric motor drive device may measure the current
flowing through the electric motor during the drive operation of the
electric motor and use that measurement current as a basis or use that
measurement current and the Vdc or other measurement voltage as the
basis for selection of the control method used. In this case, for
example, to measure the current flowing through the electric motor, an
ammeter is provided at either the positive rail 34 or negative rail 35 of
the inverter 3 which is connected to one pole of the DC power supply 2.
For example, if referring to FIG. 4 again, the ammeter 8 is provided at
the negative rail 35. Further, the ammeter 8 notifies the measured value
of the current to the control circuit 7. The control unit 13 of the
control circuit 7 acquires the current value measured by the ammeter 8
through the sensor signal receiving unit 11. If the power supply voltage
is substantially equal to the induced voltage VE (in operational
flow chart shown in FIG. 2, step S104--Yes), the control unit 13 selects
field weakening control if the measured current value is larger than a
preset predetermined value while selects voltage boosting control if the
measured current value is that predetermined value or less. Note that,
this predetermined value is, for example, set to the lower limit value
where the energy conversion efficiency when field weakening control is
applied becomes higher than the energy conversion efficiency when voltage
boosting control is applied. Furthermore, regardless of the relationship
of VL and VH and the induced voltage VE, the control unit
13 selects field weakening control if the measured current value is
larger than a preset predetermined value, while selects voltage boosting
control if the measured current value is that predetermined value or
less. In this way, by the control unit using the value of the current
flowing through the electric motor to determine the control method of the
electric motor, the control unit can select the optimal control method in
real time.

[0061]Furthermore, in the above embodiments, the control unit may correct
the estimated value of the induced voltage in accordance with status
signals showing the operating environment of the electric motor. Note
that, as a status signal expressing the operating environment, for
example, the inside temperature of the electric motor acquired by a
temperature sensor set inside the electric motor or the temperature
around the electric motor or the thermistor temperature of the inverter
acquired by a temperature sensor detecting the temperature around the
electric motor or of the thermistor of the inverter may be used. Further,
as another example of the status signal, the temperature acquired by a
temperature sensor installed inside or around the apparatus in which the
electric motor is assembled, for example, the inside temperature of an
air-conditioning system in which the electric motor is housed or the
outside air temperature acquired by an outside air temperature sensor
installed near the engine compartment in which the electric motor is
provided may be used. In this case, the interface unit of the control
circuit has the function as a status signal acquiring unit, for example,
acquires temperature information as the status signal from the
air-conditioning system or outside air temperature sensor and transfers
that status signal to the control unit.

[0062]To determine the correction value, for any of the above status
signals or combination of status signals, the mechanical angular speed of
the electric motor, the voltage Vdc across the terminals of the
capacitor, the power supply voltage, and the value of the current flowing
through the electric motor are measured in advance for various status
signal values so as to find the induced voltage for the combinations of
status signal values and mechanical angular speed of the electric motor.
Further, the difference of the induced voltage calculated by formula (1)
from the measured value is made the correction value. Further, the found
correction value is made an indicator utilized for estimation of the
induced voltage and a map linking the indicator values and the
combinations of status signal values and mechanical angular speed of the
electric motor is prepared. That map is stored in advance in the memory
of the control unit. When estimating the induced voltage during a drive
operation of the electric motor, the control unit acquires the mechanical
angular speed designated by the speed instruction signal of the electric
motor and the status signal value. Further, the control unit refers to
the map stored in the memory so as to determine the indicator value
corresponding to that mechanical angular speed and status signal value,
that is, correction value. Further, the control unit adds that correction
value to the induced voltage calculated by formula (1) to thereby find
the estimated value of the induced voltage. Alternatively, it may use
induced voltages measured for combinations of the status signal values
and mechanical angular speeds of the electric motor themselves as
indicator values and have those indicator values stored in a map. In this
case, when estimating the induced voltage during drive operation of the
electric motor, the control unit refers to the map stored in the memory
and thereby uses the value of the induced voltage corresponding to the
mechanical angular speed and status signal value of the electric motor as
the estimated value of the induced voltage.

[0063]Furthermore, by applying for example the least square method to the
measured values of the induced voltage for a combination of the status
signal values and mechanical angular speed of the electric motor, a
function expressing the relationship of the combination of the status
signal values and the mechanical angular speed of the electric motor with
the induced voltage may be determined in advance. In this case, when
estimating the induced voltage during drive of the electric motor, the
control unit determines the induced voltage by inputting the mechanical
angular speed of the electric motor shown by the speed instruction signal
and the status signal value into the above function.

[0064]Further, the above embodiments, the control circuit estimates the
induced voltage from the speed instruction signal, but the control
circuit may also calculate the induced voltage from the voltage
instruction for driving the electric motor. Note that, the voltage
instruction for driving the electric motor expresses the sum of the
induced voltage and the potential difference generated from the
resistance of the electric motor and impedance due to the coils.
Therefore, the control circuit may use the measured value of the current
flowing through the electric motor or the speed of the electric motor
calculated from the speed instruction signal or speed of the electric
motor obtained from the speed sensor as the basis to calculate the
potential difference generated due to the impedance. Further, the control
circuit can subtract the potential difference generated from the
impedance from the voltage value expressed in the voltage instruction so
as to find the induced voltage. Further, the potential difference
generated from the impedance is extremely small. For this reason, the
control circuit may ignore the potential difference arising due to the
impedance as error and use the voltage expressed by the voltage
instruction itself as the induced voltage.

[0065]Further, in the above embodiments, the control circuit may execute
the processing of steps S104 to S108 in the operational flow chart of
FIG. 2 and therefore select either of the control methods only when the
induced voltage VE is larger than VL or VH and may drive
the electric motor in accordance with a control method applied in the
past in other cases. Due to this, the control circuit can lighten the
processing load required for selecting voltage boosting control or field
weakening control.

[0066]The electric motor drive devices according to the above embodiments
or their modifications and electric motors controlled by those electric
motor drive devices can be utilized for various apparatuses, such as
compressors, blower fans, and other fan devices of air-conditioning
systems, oil pumps used for circulation of cooling water, and drive
sources or steering apparatuses of vehicles. FIG. 7 is a schematic view
of the configuration of a vehicular air-conditioning system having an
electric compressor driven by an electric motor controlled by an electric
motor drive device according to any of the embodiments of the present
invention. The vehicular air-conditioning system 100 has a heat pump
circuit 110 and a control device 120. The heat pump circuit 110 has an
electric compressor 111, condenser 112, receiver 113, expansion valve
114, and evaporator 115.

[0067]The electric compressor 111 may, for example, be made a variable
capacity type compressor having a shaft 101, a compression unit 102 to
which the shaft 101 is rotatably attached, an electric motor 103, and an
electric motor drive device 104. The electric compressor 111 transmits
the power of the electric motor 103 through a belt or pulley to the shaft
101 so as to make the shaft 101 turn. Further, by a cylinder (not shown)
attached to the shaft 101 via a swash plate (not shown) moving in the
compression unit 102, the refrigerant supplied into the compression unit
102 is compressed and becomes high pressure gas. Further, this
compression unit 102 is an operating unit which performs predetermined
work by using the power from the electric motor 103. The electric motor
103 is a multi-phase AC electric motor. In the same way as the AC
electric motors of the above embodiments, the coils of the different
phases are connected in a star configuration. Further, between the
neutral point of the electric motor 103 and the positive rail or negative
rail of the inverter of the electric motor drive device 104, DC electric
power from a DC power supply (not shown) is supplied, whereby the
electric motor 103 turns by that DC electric power. Note that, the DC
power supply, for example, may be made a battery carried in a vehicle or
a power supply circuit converting voltage supplied from a battery to a
predetermined voltage and outputting the converted voltage. The electric
motor drive device 104 is an electric motor drive device of any of the
above embodiments. Further, the electric motor drive device 104 selects
either voltage boosting control or field weakening control based on the
induced voltage generated at the electric motor 103 and the voltage
across the terminals of the capacitor of the electric motor drive device
104 and drives the electric motor 103 in accordance with the selected
control method.

[0068]The condenser 112 cools the high temperature, high pressure
refrigerant gas sent from the electric compressor 111 and causes it to
liquefy. The receiver 113 stores the liquefied refrigerant gas. Further,
the receiver 113 prevents a drop in cooling performance by removing the
gas bubbles contained in the liquefied refrigerant and sending only
completely liquefied refrigerant to the expansion valve 114. The
expansion valve 114 causes the liquefied refrigerant to expand by
adiabatic expansion to lower the temperature and lower the pressure and
sends the result to the evaporator 115. The evaporator 115 performs heat
exchange between the low temperature, low pressure refrigerant and the
air blown into the evaporator 115 by a blower fan (not shown) etc. to
cool that air. The cooled air is blown out into the cabin and
air-conditions the cabin. On the other hand, the refrigerant warmed by
heat exchange at the evaporator 115 again flows into the electric
compressor 111.

[0069]In the same way as the electric compressor 111, the blower fan is
also a device driven by the AC electric motor. The blower fan transmits
the drive power supplied by the AC electric motor through a direct drive
connecting shaft to the operating part, that is, the fan, so as to make
the fan turn. In this blower fan as well, by using the electric motor
drive device according to any of the embodiments of the present invention
to control the AC electric motor, it becomes possible to drive the
electric motor with a good energy conversion efficiency.

[0070]The control device (hereinafter referred to as the "air-conditioning
ECU") 120 is comprised of a built-in type microprocessor, memory,
communication circuit, and its peripheral circuits. Further, the
air-conditioning ECU 120 controls the parts of the air-conditioning
system 100 in accordance with a program operating on the microprocessor,
target room temperature, cabin inside air temperature, etc. Further, the
air-conditioning ECU 120 can communicate through a control area network
(CAN) or other such car mounted network with an electric motor drive
device 104 according to any of the above embodiments for controlling the
electric motor 103. Further, the air-conditioning ECU 120 determines the
speed of the electric motor 103 (mechanical angular speed) in accordance
with the target room temperature and cabin inside air temperature.
Further, the air-conditioning ECU 120 notifies that speed to the electric
motor drive device 104.

[0071]In the above way, a person skilled in the art could make various
changes to the embodiments within the scope of the present invention.